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High force optical tweezers

Optical tweezers are really useful tools because a particle trapped in a beam can be modeled as a particle sitting in a harmonic potential well – or more simply it can be modelled as though the particle is attached to a spring which always acts to pull the particle back to the most intense part of the beam. In this way we can use Hooke’s Law, F = -kx to describe the particle motion, where is the particle displacment from the equilibrium (zero force) position and k is called the spring constant and is a measure of the ‘stiffness’ of the spring. Typically optical tweezers work well in the low picoNewton range, which is great for looking at single molecule force production and also at some cellular forces, but for many cell migration issues these simply aren’t enough.

Recently there have been attempts to try and increase the force regime in which tweezers work – one has looked at modifying the refractive index properties of the beads and  another, which I’ll discuss here, has looked at modifying the surface under which the particles are trapped. This paper, from a group based at the National Technical University of Athens, published in Applied Physics Letters, considers surfaces made of silicon which are irradiated using high energy laser pulses. This irradiation leads to the surface becoming roughened and the production of little silicon pillars. These have been used in the past to try and develop single electron emitters (for things like a single pixel CRT display). In this experiment the roughened surface is then covered by a thin layer of silver. If you then illuminate this metallic substrate – here with a focussed tweezers trapping beam – you excite surface plasmons (or at least this is the suggested mechanism for the enhancement) in the metal, and these result in an enhanced field around the pillar tips. This enhancment, according to the authors, results in an optical field that can trap a 900nm diameter silica bead with a force of 4.3nN, which is anything up to 1000x stronger than a conventional trap. Pretty impressive.

Surface of a silicon wafer after laser processing produces a roughened surface made from a series of pillars. Image from Appl. Phys. Lett. 2012 vol. 101 (1) p. 011102 (2012) DOI: 10.1063/1.4728992

What’s also interesting about this idea is that typically plasmonic enhancements result in field increases very close to the surface, and as this can limit the size of the particles that can be trapped, and often necessitates that the particles must touch the surface. Here, because the geometry makes use of a conventional tweezers beam illuminating the surface from above, it appears that the trapped particle can be an appreciable distance (well at least 1 micrometer) away from the surface. This could open up the technique to possible uses in biological force sensing, although the silver film could prove toxic – more experiments are clearly needed to check this. It may also have applications in areas such as monitoring fluid flow and higher pressure material interactions than can be easily dealt with by low force tweezers. The last nice thing about the technique is that it’s not too difficult to do, provided you have some expensive lasers to hand, and this type of materials processing is one of things that people here in my department in Dundee work on, so we should be able to give it a try and look at the possible applications.

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